To increase the storage capacity of an optical disk, a laser beam for reading and/or writing data from/on it should have a shortened wavelength. Most of DVD players and recorders currently on the market use red semiconductor lasers operating at wavelengths of around 660 nm. A red semiconductor laser like this is fabricated by epitaxially growing an InGaAlP based compound semiconductor on a GaAs wafer, for example.
Recently, people are spending a lot of energy in developing next-generation optical disks that have greater storage capacities than DVDs. A light source for each of those next-generation optical disks needs to constantly radiate a violet laser beam (falling within the wavelength range of around 400 nm), of which the wavelength is even shorter than that of the red ray. A GaN based semiconductor laser, operating at wavelengths of around 400 nm, is one of the most promising light sources for reading from and writing to a Blu-ray Disc™ and other next-generation optical disks. However, the semiconductor laser still has some hurdles to clear to be commercially viable products.
The light-current characteristic curve of a GaN based semiconductor laser should have no kinks in a high optical output range. A kink is observed on the light-current characteristic curve when the laser diode has unstable horizontal transverse mode. That is why a laser structure that can stabilize the horizontal transverse mode should be realized.
Meanwhile, the GaN based semiconductor is not only made up of hard crystals but also chemically stable, and therefore, it is difficult to pattern this material by a wet etching technique. Accordingly, a ridge structure, which is needed to control the horizontal transverse mode, is formed by patterning a GaN based semiconductor layer by a dry etching process. It was reported that a GaN based laser diode with a ridge structure formed by a dry etching process achieved continuous-wave oscillation at room temperature (see IEEE Journal of Selected Topics in Quantum Electronics, Vol. 4 (1998), pp. 483-489 and Japanese Journal of Applied Physics, Vol. 41 (2002), pp. 1829-1833).
However, the fine-line patterning process of a GaN based semiconductor by a dry etching technique is too difficult to control to advance the etching process to a uniform depth within a wafer plane. If the etch depth changed from one location on the wafer to another, then the horizontal transverse mode would not be stabilized among a number of semiconductor lasers that have been cut out from the same wafer. In that case, some elements might have light-current characteristic characteristics with kinks. On top of that, the process reproducibility would decrease not just within the wafer plane but also from one processing lot to another, thus decreasing the production yield of GaN based laser diodes and raising the manufacturing cost instead.
A technique of forming a ridge structure for the GaN based laser diode by selective regrowth was proposed recently (see Japanese Journal of Applied Physics, Vol. 40 (2001), L925 through L927). According to the method proposed in this document, after a number of semiconductor layers have been stacked on an active layer, the wafer is entirely covered with an SiO2 film except a portion to be the ridge structure. Thereafter, the crystal growing process is carried out for the second time, thereby selectively re-growing the semiconductor layers on that portion not covered with the SiO2 film and defining the ridge structure. This method makes it possible to form the ridge structure without patterning the semiconductor layers by a dry etching process. Thus, a manufacturing process with excellent uniformity and reproducibility is provided. In addition, it is also possible to avoid doing any damage on the active layer as a result of the dry etching process.
Nevertheless, according to such a selective regrowth process, it is difficult to avoid depositing a lot of GaN based poly-crystals (poly structure) on the masking SiO2 film. For that reason, if the laser diodes are mounted in a junction-down arrangement to increase the heat dissipation when the optical output of the laser diodes is increased, then the laser diodes will make a less close contact with either a heat sink or a sub-mount, thus causing some inconveniences such as fixing failures.
Besides, since the surface of the crystals is covered with the SiO2 film except the portion to be the ridge structure, the resultant laser diodes have poor heat conduction and heat dissipation and a shorter life.
In order to overcome the problems described above, a primary object of the present invention is to provide a semiconductor light-emitting element that has a novel current confining structure and that does not need any ridge structure for current confining purposes.
Another object of the present invention is to provide a semiconductor light-emitting element that achieves excellent horizontal transverse mode control and heat dissipation, exhibits no kinks even when operated with its optical output increased, and has an extended life.
Yet another object of the present invention is to provide a method for fabricating such a semiconductor light-emitting element at a high yield and a reduced cost.